This invention relates to a method for controlling drive of an injector for an internal combustion engine and an apparatus therefor.
In general, an injector has been conventionally used for the purpose of feeding an internal combustion engine with fuel, which includes a cylinder provided at a distal end thereof with a fuel injection port, an injection valve for operating or selectively opening the injection port and a solenoid coil fed with a drive current when the injection valve opens the fuel injection port. Fuel is fed into the cylinder from a fuel tank under a pressure of a predetermined level by means of a fuel pump.
The injector is so arranged that the fuel injection port communicates with both an intake manifold of the internal combustion engine and a fuel injection space defined in a cylinder of the engine which is a space defined in the cylinder into which fuel is to be injected. The injector thus arranged functions to permit the injection valve to open the fuel injection port, resulting in injection of fuel when a drive current of a predetermined level is fed to the solenoid coil.
A fuel injection rate or a rate at which fuel is injected from the injector is generally determined depending on a pressure under which fuel is fed from the fuel pump and a period of time for which the fuel injection port is kept open by the injection valve. In general, a pressure of fuel fed to the injector is controlled to be constant by means of a pressure regulator, so that a rate of fuel injected from the injector depends on a period of time during which the injection valve keeps the fuel injection port open.
A unit for driving the thus-constructed injector generally includes a drive current detection circuit for detecting a drive current flowing through the solenoid coil to generate a drive current detection signal, an indication signal generation circuit for generating an indication signal providing an indicated value for the drive current, and a current feed control circuit for controlling current feed to the solenoid coil so as to render the drive current equal to the indicated value. The current feed control circuit permits a drive current corresponding in magnitude to the indication signal to be flowed through the solenoid coil during a period of time for which it is fed with a drive pulse like a rectangular wave for commanding injection of fuel while using the drive current detection signal and indication signal as an input therefor.
In order to improve characteristics for controlling a rate at which fuel is fed to an internal combustion engine by means of the injector, it is desired to increase a dynamic range of the injector as much as possible, to thereby increase a width of adjustment of the fuel injection rate. The dynamic range used herein indicates a ratio (qmax/qmin) between a maximum fuel injection rate (qmax) and a minimum one (qmin).
In the art, a saturated system and a peak hold system have been known as a method for driving the injector constructed as described above.
The saturated system is adapted to connect a switch element such as a transistor or the like in series to the solenoid coil to provide the switch element with a drive pulse like a rectangular wave while setting a resistance value of a current feed circuit of the solenoid coil at a relatively high level as much as about 12 .OMEGA.. The switch element is kept turned on while it is fed with the drive pulse, resulting in applying a power voltage of a constant level to the solenoid coil. Such application of the power voltage to the solenoid coil gradually increases the drive current flowing through the solenoid coil, to thereby render the injection valve open when the drive current reaches a valve opening current level. Then, the drive current converges to a saturated value determined depending on both an impedance of the current feed circuit and the power voltage and is kept at the saturated value until the drive pulse is extinguished. Such extinction of the drive pulse causes the drive current to be rendered zero.
The saturated system thus constructed simplifies construction of the drive circuit, leading to a reduction in manufacturing cost of the drive control unit; however, it causes the drive current to be kept at the saturated value for a relatively long period of time, to thereby cause an increase in power consumption, resulting in an increase in generation of heat therefrom.
In the injector of the electromagnetic type which is adapted to drive the injection valve by means of the solenoid, it is required to flow a relatively large current as high as a valve opening current value or more through the solenoid coil when the injection valve opens the fuel injection port. The valve opening current value is an inherent or intrinsic value determined depending on the injector. However, when it is desired that the fuel injection port is subsequently kept open once it is opened, it is merely required to flow a hold current of a level lower than the valve opening current value therethrough. Thus, the peak hold system, as disclosed in Japanese Patent Publication No. 1259/1983 and Japanese Patent Application Laid-Open Publication No. 287850/1992, is so constructed that the drive current is rapidly increased to a peak value above the valve opening current value when the drive pulse is applied to the solenoid coil and then reduced to a hold value required to hold the fuel injection port open, to thereby hold it at the hold value until the drive pulse is extinguished, while setting a resistance value of the solenoid coil at a level as low as about 2 .OMEGA..
Thus, the peak hold system permits the drive current to be reduced to the hold value after opening of the fuel injection port, leading to a reduction in power consumption, resulting in heat generation being minimized. Also, it decreases a port opening period or a period of time during which the fuel injection port is kept open, so that the maximum fuel injection rate or quantity when a cycle of generation of the drive pulse is rendered constant may be increased. Thus, the peak hold system increases a dynamic range of the injector as compared with the saturated system.
Conventionally, driving of the injector according to the peak hold system is carried out in a manner to apply a fixed drive voltage stepwise rising to the solenoid coil to increase a drive current flowing through the solenoid coil toward a peak value, stepwise reduce a drive voltage to a low level to attenuate the drive current to the hold value after the drive current reaches the peak value, and then decrease the drive voltage to a zero level to naturally attenuate the drive current to a zero level when the drive pulse is extinguished.
In the case that the drive voltage is varied as described above, the injection valve starts operation of opening fuel injection port (hereinafter also referred to "port opening operation") when a predetermined length of port opening time elapses after application of the drive pulse, so that the injection valve opens the fuel injection port at certain time. When the drive voltage is reduced to a zero level at the time when the drive pulse is extinguished, the drive current is naturally attenuated, resulting in being reduced to a zero level in a short period of time. Irrespective of such a reduction of the drive current to a zero level, the fuel injection port is kept open for a certain period of time due to a residual magnetic flux of the solenoid coil, so that the injection valve starts operation of closing the fuel injection port (hereinafter also referred to as "port closing operation") when a predetermined period of lag time elapses after the drive voltage is reduced to a zero level.
Control of a fuel feed rate or a rate at which fuel is fed to the internal combustion engine by means of the injector is carried out by varying a pulse width of the drive pulse to vary a port opening period of the injection valve, to thereby vary the fuel injection rate. In this instance, in order to improve the control characteristics, it is desired to increase a dynamic range of the injector as much as possible, to thereby increase an adjustment width of the fuel injection rate.
Unfortunately, techniques wherein a voltage across the solenoid coil is stepwise decreased to naturally attenuate the drive current to the hold value when the drive current is shifted from the peak value to the hold value as in the prior art reduces a pulse width of the drive pulse due to a decrease in fuel injection rate, resulting in the port closing operation of the injection valve being started at identical time irrespective of a length of the pulse width of the drive pulse (or irrespective of time at which the drive pulse is rendered zero) when the drive pulse is rendered zero during shift of the drive current from the peak value to the hold value. This fails to permit a fuel injection quantity per one drive pulse to be varied in correspondence to a variation in pulse width of the drive pulse. Such a failure in variation in fuel injection rate or quantity in correspondence to a variation in pulse width of the drive pulse leads to a failure in control of the fuel injection rate, so that control of the fuel feed rate based on a variation in pulse width of the drive pulse requires to restrict a lower limit value of the pulse width of the drive pulse in order to avoid such a situation as described above. Thus, employment of the conventional drive procedure in control of the injector according to the peak hold system causes an increase in minimum injection rate available for the control, leading to a decrease in dynamic range of the injector, resulting in a reduction in adjustment range of the fuel injection rate during control of the fuel injection rate, so that the control characteristics for the fuel injection rate are deteriorated.